The present invention relates to semiconductor packages, and more particularly, to a semiconductor package with a heat sink having air vents.
A ball grid array (BGA) semiconductor package is the mainstream in packaged products for providing high density of electronic components and electronic circuits mounted therein with sufficient I/O connections, and accordingly array-arranged solder balls connected to the I/O connections are increased in number and density. As a result increase in heat generated in operating the semiconductor package can be expected, and the generated heat then requires to be dissipated effectively, otherwise, the reliability and life-time of the semiconductor package may be detrimentally affected.
Besides, in the foregoing semiconductor package, materials used for making a substrate for mounting a semiconductor chip thereon and for making an encapsulant for encapsulating the semiconductor chip are also critical in consideration of the heat dissipation. For example, in a plastic BGA (PBGA), ceramic BGA (CBGA) or tape BGA (TBGA) semiconductor package, the substrate is widely made of a resin or ceramic material having the heat dissipating efficiency much poorer than a metallic material. Further, the encapsulant is made of a molding resin having a sufficiently small coefficient of conductivity approximately only 0.8 w/mxe2x96xa1k, which makes the heat generated from the semiconductor chip with electron circuits and electronic elements mounted thereon not able to be dissipated to the atmosphere effectively, and thus the reliability of the semiconductor chip is degraded. Therefore, a BGA semiconductor package having a heat sink of god thermal conductivity is disclosed in U.S. Pat. Nos. 5,851,377 and 5,977,626 for improving the heat dissipating efficiency.
Illustrated in FIG. 1 is the conventional semiconductor package 2 having the heat sink 23. A semiconductor chip 21 is attached to a substrate 20 by means of an adhesive 212 such as silver paste, and then is electrically connected to the substrate 20 through gold wires 22. Subsequently, the heat sink 23 is mounted at pre-determined positions on the substrate 20 in no contact with the semiconductor chip 21 and the gold wires 22 positioned below a flat portion 230 of the heat sink 23, wherein the flat portion 230 downwardly extends to form supporting members 231 for strongly supporting the flat portion 230 on the substrate 20. Thereafter, a molding process is performed to form an encapsulant 24 for encapsulating the semiconductor chip 21, the gold wires 22 and the heat sink 23, while the flat portion 230 of the heat sink is partially exposed to the outside of the encapsulant 24. Finally, with the implantation of solder balls 25, the semiconductor package 2 is completed in fabrication.
The fabricated semiconductor package 2 is of a thickness approximately 1.17 mm, wherein, excluding the thinness of the semiconductor chip 21, the adhesive 212 and the heat sink 23, a 0.54 mm gap d is remained for filing the encapsulant 24 therein, that is, the heat generated from the semiconductor chip 21 needs to pass a 0.54 mm-long path through the encapsulant 24 before reaching the heat sink 23 to be dissipated to the atmosphere. However, as the encapsulant 24 is rather poor in thermal conduction, which makes the heat dissipating efficiency reduced, and further results in decrease in the reliability of the semiconductor chip. Therefore, in the fabrication of the semiconductor package, it tends to shorten the heat dissipating path i.e. the gap d, so as to improve the heat dissipating efficiency and reduce the overall thickness of the semiconductor package as well as save the packaging cost.
However, the shortening for the heat-dissipating path is restricted in accordance with the application of the heat sink embedded in the above semiconductor package, and accordingly problems are possibly generated.
First, during the molding process, as shown in FIG. 2, a molten resin 34 flows into a runner 36 formed by molds 37 and a substrate 30 clamped by the molds 37, and then through an injecting gate 38 into a mold cavity 372. As the molten resin 34 flows from the injecting gate 38 into the relatively wider mold cavity 372, the resin flow rate increases; then while the molten resin 34 further flow to a gap 39 between a semiconductor chip 31 and a heat sink 33, the flow rate slows down since the gap 39 is narrower than the mold cavity 372. This results in an unstable resin flow rate as the molten resin 34 flows faster at the outside of the gap 39 than inside of the gap 39, as illustrated as dotted lines in a resin flow curve of FIG. 3. Due to the unstable flow rate, air in the gap 39 can not be escaped through an air channel 373, and is trapped in the gap 39 to form voids. The void formation will increase the heat resistance, and accordingly the heat dissipating efficiency will be reduced. Moreover, a popcorn effect will also be generated as proceeding subsequent processes for fabricating the semiconductor package under a high temperature due to the void formation, and thus the semiconductor package will be damaged by the popcorn effect and the relability thereof is definitely degraded.
In addition, further in the semiconductor package with the heat sink tightly bonded to the encapsulant, a thermal stress is generated due to temperature variations in a reflow solder process and a temperature cycle of a reliability test, according to the difference in coefficient of thermal expansion (CTE) between the heat sink and the encapsulant. This makes the heat sink and the encapsulant differently deformed, resulting in warpage and delamination for degrading the reliability of the semiconductor package.
A objective of the present invention is to provide a semiconductor package with a heat sink having taper air vents for ventilating air in a mold cavity, and for avoiding voids formed in an encapsulant in a gap between a semiconductor chip and the heat sink during a molding process. Moreover, the semiconductor package of the invention is capable of dispersing a thermal stress on the encapsulant and the heat sink for preventing warpage and delamination from occurrence, so as to assure the reliability of the semiconductor package.
According to the foregoing and other objectives, the semiconductor package of the present invention comprises: a substrate having a first surface and a second surface; a semiconductor chip having a first surface and a second surface, while the second surface of the chip is attached to the first surface of the substrate; a plurality of first conductive elements (i.e. gold wires) for electrically connecting the chip to the substrate; an embedded heat sink mounted on the first surface of the substrate and having a flat portion and a plurality of supporting members, while the supporting members are used to support the flat portion of the heat sink to be positioned above the chip without contacting the first conductive elements; a plurality of second conductive elements (i.e. solder balls) mounted on the second surface of the substrate for electrically connecting the chip to an external device; and an encapsulant for encapsulating the chip, the first conductive elements and the heat sink while the flat portion of the heat sink is partially exposed to the outside of the encapsulant.
The invention is further characterized in forming at least one taper air vent in the flat portion of the heat sink, wherein the taper air vent has a wide opening facing the semiconductor chip and a narrow opening exposed to the outside of the semiconductor package. During a molding process, air in a gap between the heat sink and the chip allows to be escaped through the taper air vents so as to prevent voids from forming in the encapsulant. As the result, the semiconductor package can be improved in heat dissipating efficiency, and a popcorn effect can be prevented from occurring in the encapsulant, as well as the yield of the packaged products can be raised.
On the other hand, unlike a conventional semiconductor package having an embedded heat sink tightly bonded to an encapsulant, the semiconductor package of the invention is formed with at least one air vent in the flat portion of the heat sink, allowing a thermal stress generated by the heat sink during molding to be dispersed as well as planarity of the flat portion of the heat sink to be maintained.